1. Field of Invention
The invention relates to an optical lens, more particularly to an optical lens having a modified reflection surface to direct light and a backlight module including the optical lens.
2. Description of the Related Art
Currently, a thin film transistor liquid crystal display (TFT-LCD) has been the main stream of a flat panel display with a liquid crystal module. Since the TFT-LCD is a non-active light-emitting display, a backlight module and a color filter are usually needed to respectively provide relatively uniform light brightness and more plentiful colors.
Referring to FIG. 1, a conventional direct-type backlight module includes a plurality of light sources 91 and a film substrate 92. Since a light-emitting diode (LED) has a relatively small size and low energy consumption, the LED may be used as the light source 91 of the conventional direct-type backlight module. However, since the LED emits light in a Lambortian distribution, a relatively small-light spot is formed on the film substrate 92 right above the LED. Therefore, the conventional direct-type backlight module needs a greater number of LEDs as the light sources 91 for increasing the illumination area.
Referring to FIG. 2, each of the light sources 91 is additionally covered by an optical lens 90 that is also called a secondary optical lens to vary the light distribution of the light emitted from the light source 91. The optical lenses 90 may increase the light scattering angles of the light emitted from the light sources 91, thereby reducing the height required for light mixing. With the addition of the optical lenses 90, the conventional backlight module can form a relatively larger illumination area and the number of the light sources 91 used is decreased so as to save the fabrication cost.
In order to increase the size of the light spot right above the light source 91 on the film substrate 92, a light-entering surface 93, a light-exiting surface 94, and a base surface 95 of the optical lens 90 are configured as described below, and as illustrated in FIG. 3. The light-exiting surface 94 has a substantially curved configuration. The base surface 95 has a substantially planar configuration. When the light (A) emitted from the lighting source 91 enters the base surface 95 and then exits the light-exiting surface 94, a part of the light (A) is reflected from the light-exiting surface 94 to the base surface 95. When the part of light (A) is reflected once again back to the light-exiting surface 94 from the base surface 95, the part of light (A) moves toward a central axis (N1) of the optical lens 90, creating the relatively high brightness around the central axis (N1), which results in an uneven light distribution on the film substrate 92.